1. Field of the Invention
The present invention relates to a backlight unit for a liquid crystal display (LCD) device, and more particularly, to a lamp guide, a backlight unit including the lamp guide and a liquid crystal display device including the backlight unit.
2. Discussion of the Related Art
Although cathode-ray tubes (CRTs) have been widely used as a display device for a television, a measuring instrument or an information terminal, it is hard to apply the CRTs to reduced electronic products in size and weight because of their heavy weight and large volume. Recently, flat panel display (FPD) such as liquid crystal display (LCD) devices, plasma display panels (PDPs), electroluminescent display devices (ELDs) and field emission display (FED) devices have been the subject of research and development because of their superior thin profile and light weight as compared with the CRTs.
Among various display devices, LCD devices have been widely used for a notebook computer, a monitor and a television because of their superiority in display quality of moving images and contrast ratio. The LCD device includes a liquid crystal panel having opposite two substrates and a liquid crystal layer between the two substrates. Since the LCD device is a non-emissive display device that displays images using an external light, the LCD device further includes a backlight unit under the liquid crystal panel as an additional light source.
The backlight unit is classified into an edge type and a direct type according to a position relation between the liquid crystal panel and a lamp of the backlight unit. The edge type backlight unit includes a light guide plate and at least one lamp disposed at one side or both sides of the light guide plate. The direct type backlight unit includes a plurality of lamps disposed under a plurality of optic sheets. In addition, the direct type backlight unit does not require a light guide plate (LGP) reflecting and refracting the light and has no limit according to a display size of the liquid crystal panel. Accordingly, the direct type backlight unit has been widely used for an LCD device having a size over about 20 inches.
FIG. 1 is a cross-sectional view showing a liquid crystal display device including a direct type backlight unit according to the related art. In FIG. 1, a liquid crystal panel 10 and a backlight unit 20 are integrated in an LCD device by mechanical elements. Accordingly, the LCD device includes the liquid crystal panel 10, the backlight unit 20, a bottom frame 50, a main frame 30 and a top frame 40. The liquid crystal panel 10 is disposed over the backlight unit 20, and the main frame 30 surrounds side surfaces of the liquid crystal panel 10 and the backlight unit 20. The top frame 40 surrounds a front edge surface of the liquid crystal panel 10, and the bottom frame 50 wraps a rear surface of the backlight unit 20. The top frame 40 and the bottom frame 50 are combined with the main frame 30.
The liquid crystal panel 10 includes a first substrate 12, a second substrate 14 and a liquid crystal layer (not shown) between the first and second substrates 12 and 14. Although not shown in FIG. 1, a driving circuit is connected to a side of the liquid crystal panel 10 and is bent toward a rear of the liquid crystal panel 10.
The backlight unit 20 includes a reflecting sheet 22, a plurality of lamps 24, a diffusing plate 29 and a plurality of optic sheets 26. The reflecting sheet 22 is disposed over the bottom frame 50. The plurality of lamps 24 are arranged in parallel over the reflecting sheet 22, and the diffusing plate 29 covers the plurality of lamps 24. The plurality of optic sheets 26 are disposed over the diffusing plate 29. Although not shown in FIG. 1, the plurality of lamps 24 may be fixed and supported by a lamp guide. The lamp guide supports the diffusing plate 29 and the plurality of optic sheets 26 to keep a uniform gap distance between the diffusing plate 29 and the plurality of lamps 24. In addition, the lamp guide prevents sway and break of the plurality of lamps 24 by impact.
FIG. 2 is a cross-sectional view showing a direct type backlight unit of a liquid crystal display device according to the related art. In FIG. 2, a reflecting sheet 22 and a lamp guide 60 are sequentially disposed over a bottom frame 50. The lamp guide 60 is coupled with the bottom frame 50 through a plurality of through holes of the reflecting sheet 22 and the bottom frame 50. A plurality of lamps 24 are hold and fixed by the lamp guide 60. In addition, a diffusing plate 29 and a plurality of optic sheets 26 are disposed over the plurality of lamps 24.
The lamp guide 60 includes a horizontal part 62, a supporting part 68, a holding part 66 and a coupling part 64. Since the supporting part 68 having a cone shape upwardly extends from a front surface of the horizontal part 62 to support the diffusing plate 29 and the plurality of optic sheets 26, the gap distance between each lamp 24 and the diffusing plate 29 is kept uniform. The holding part 66 on the front surface of the horizontal part 62 holds and fixes each lamp 24. In addition, the holding part 66 has a ring shape having an open portion. The coupling part 64 downwardly extends from a rear surface of the horizontal part 62 and is inserted through the plurality of through holes of the reflecting sheet 22 and the bottom frame 50.
As a size of the LCD device is enlarged, a thin profile of the LCD device has been the subject of recent research and development. For example, the LCD device having a reduced first distance d1 between the diffusing plate 29 and each lamp 24 and a reduced second distance d2 between the reflecting sheet 22 and each lamp 24 has been suggested. However, the LCD device has disadvantages due to the reduced first and second distances d1 and d2. For example, the light from the backlight unit may have a shadow band of the lamp guide 60. When the light from each lamp 24 is diffused by the diffusing plate 29, a plurality of virtual images separated from each other is generated due to a total reflection at a front surface of the diffusing plate 29. Specifically, the holding part 66 of the lamp guide 60 wrapping and blocking a portion of an outer surface of each lamp 24 generates the corresponding virtual images. Since the horizontal part 62 of the lamp guide 60 is arranged along a vertical line, the virtual images corresponding to the blocked portions of the adjacent lamps 24 held by the holding parts 66 of the lamp guide 60 overlap each other and form a shadow band having a relatively low luminance due to a light loss. As a result, the shadow band causes a stain of a displayed image of the LCD device.
FIG. 3 is a view showing a luminance distribution image of a backlight unit according to the related art. In FIG. 3, since the virtual images corresponding to the adjacent lamps 24 (of FIG. 2) overlap each other, a shadow band is generated in the luminance distribution image of the backlight unit and a display quality of the LCD device including the backlight unit is reduced.